1. Field of the Invention
This invention relates generally to wheelchairs, and more particularly to a wheelchair of the type in which the main wheels are located essentially under the center of gravity. Such wheelchairs are generally referred to as center of gravity wheelchairs. The present invention further relates to such wheelchairs having an articulated chassis whereby the seat and thus the center of gravity may be moved forwardly relative to the main wheels, thereby increasing the stability and facilitating curb climbiing. Moreover, the articulated chassis enables the rider to tilt backwards thereby achieving a change in postural support. Further, the wheelchair of the invention has caster wheels fore and aft thereof arranged in a diamond pattern with the main wheels so that lateral side drift of the wheelchair is minimized when traversing sloping surfaces.
2. Prior Art
Many different wheelchair designs are known in the prior art, including so called center of gravity wheelchairs and wheelchairs having articulated chassis and fore and aft caster wheels. Some prior art wheelchairs even have lever operated front and rear caster wheels which can be lifted to facilitate curb climbing. Other wheelchair designs have small wheels projecting fore and aft and supported on arms restrained by springs whereby the user is enabled to safely tilt back.
Examples of some of these prior art wheelchair designs are shown in U.S. Pat. Nos. 2,986,200, 3,104,112, 3,191,990, 3,580,591, 3,901,527, 3,953,054 and 4,108,449.
While these prior art wheelchair designs do provide certain advantages over conventional rigid frame wheelchairs having only a pair of main wheels and a pair of casters in front, these prior art designs possess several disadvantages. For example, wheelchairs having either front caster wheels or rear caster wheels exhibit a strong tendency to turn sideways when being propelled on a side-sloping surface. The energy for maintaining a straight course under these conditions often exceeds the energy required to propel the chair and is a serious concern for street users. Center of gravity wheelchairs such as those described herein overcome this difficulty, but have other problems. For instance, in one design with an articulated chassis, the chassis did not articulate automatically and hence all four wheels did not contact the ground under all conditions. This condition leads to instability with one main wheel off the ground. Other designs have articulated chassis constructed such that when the hand rims or main wheels are propelled the chair tilts back and forth with each stroke. Other chairs overcome these difficulties but have small caster wheels making them unsuitable for outside use. Another major disadvantage of conventional center of gravity wheelchairs is that they are difficult to force up over curbs because of the amount of weight immediately over the main wheels.